1. Field of the Invention
The present invention relates generally to a shape measuring instrument and a measuring head calibration method therefor, and more particularly to a roundness measuring instrument which measures a shape of a workpiece such as roundness and cylindricity and a measuring head calibration method therefor.
2. Description of Related Art
Examples of the roundness measuring instruments are: a table rotating type roundness measuring instrument in which an object of measurement or a workpiece is placed on a rotary table provided on a base and is measured by a fixed or vertically moved measuring head provided on a column; and a measuring head rotating type roundness measuring instrument in which the workpiece is placed on an unrotatable table and is measured by a measuring head that is rotated and vertically moved. The table rotating type roundness measuring instrument is used for small workpieces, whereas the measuring head rotating type roundness measuring instrument is used for large workpieces.
A description will be given of an example of the measuring head rotating type roundness measuring instrument. As shown in FIG. 1, an XY-table 12 is supported on a base 11 in such a way as to move in horizontal directions (XY-directions). A column 13 stands on the base 11, and a Z-table 14 is supported on the column 13 in such a way as to move in a vertical direction (Z-direction). A rotation supporting part 15 is provided on the Z-table 14, and a spindle 16, which rotates on a vertical axis, is built in the rotation supporting part 15. A supporting arm 17 is attached to the end of the spindle 16, and a measuring head holder 18 is mounted on the supporting arm 17 in such a way as to move along the supporting arm 17 (in a horizontal direction). A measuring head 19 provided with a contact 19a is attached at an end 18a of the measuring head holder 18.
To measure the workpiece in the measuring head rotating type roundness measuring instrument which is constructed in the above-mentioned manner, the XY-table 12 moves to bring the center of the measured area of the workpiece substantially to the rotational center of the measuring head 19 (within an area where the measuring head 19 is capable of measuring the workpiece). Then, the contact 19a is pressed onto the circumferential surface of the workpiece and the spindle 16 rotates the measuring head 19 to measure the shape of the circumferential surface of the workpiece, and the roundness of the horizontal section of the workpiece is calculated from the measurement data.
A description will be given of an example of the table rotating type roundness measuring instrument. As shown in FIG. 2, a rotary table 22, which rotates on a vertical axis, is provided on a base 21. A column 23 stands on the base 21, and a Z-table 24 is supported on the column 23 in such a way as to move in a vertical direction (Z-direction). A measuring head holder 25 is supported on the Z-table 24 in such a way as to move in a horizontal direction (X-direction). A measuring head 26 provided with a contact 26a is attached at an end 25a of the measuring head holder 25. The position in Y-direction of the contact 26a substantially corresponds to that of the rotational center of the rotary table 22.
To measure the workpiece in the table rotating type roundness measuring instrument which is constructed in the above-mentioned manner, the workpiece is placed on the rotary table 22 so that the center of the measured area of the workpiece is substantially on the rotational center of the rotary table 22 (within an area where the measuring head 26 is capable of measuring the workpiece). Then, the coast 26a is pressed onto the circumferential surface of the workpiece and the rotary table 22 is rotated to measure the shape of the circumferential surface of the workpiece, and the roundness of the horizontal section of the workpiece is calculated from the measurement data.
In the measuring head rotating type roundness measuring instrument and the table rotating type roundness measuring instrument, the measuring heads 19 and 26 can be moved vertically by the Z-tables 14 and 24, and thus, the roundness of the workpiece can be measured at an arbitrary height. It is therefore possible to calculate the coaxiality and cylindricity of the workpiece from the roundness data on a plurality of measured sections. It is also possible to measure the straightness of the workpiece by vertically moving the measuring heads 19 and 26 without rotating the measuring head 19 and the rotary table 22.
A differential transformer is used for the measuring heads 19 and 26. Since the differential transformer is analog, it is necessary to calibrate the measuring head prior to the measurement of the workpiece by measuring a reference gage such as a block gage so that the shape data acquired by measuring the reference gage can exactly correspond to true shape data of the reference gage. For this reason, there is the necessity for calibrating the measuring head over again if the contact 19a or 26a is changed or the surrounding conditions are changed, etc.
To calibrate the measuring head, the measuring head 19 is detached from the end 18a of the measuring head holder 18, or the measuring head 26 is detached from the end 25a of the measuring head holder 25, and then, the measuring head 19 or 26 is attached to a separately-provided attachment base.
Thus, it takes much time to calibrate the measuring heads 19 and 26 since they must be detached. In addition, there is the necessity for separately providing the attachment base, on which the measuring heads 19 and 26 are attached to be calibrated.